docs: update Terraform.gitignore (#2998)

Terraform.gitignore

@@ -24,3 +24,6 @@ override.tf.json
 # Include override files you do wish to add to version control using negated pattern
 #
 # !example_override.tf
+
+# Include tfplan files to ignore the plan output of command: terraform plan -out=tfplan
+# example: *tfplan*